Many different types of products and notably food products are subjected to a screening operation at one point or another in the processing operation. Screening may be employed as a means for grading the product according to particle dimensions, or to remove unwanted materials or contaminants.
Most screening operations deal with dry products such as flour, grains , seeds and the like. For such products a simple wire mesh screen is typically employed with mesh dimensions tailored to produce optimum results for the product type and grain size involved. The dry, solid nature of such products lends itself well to this common form of screening and little difficulty is experienced with clogging or caking of the materials being screened. There are, of course, always compromises to be made in which screening effectiveness is traded off for higher rates of product flow.
The screening of moist or wet products, and particularly of moist products having a fibrous structure, presents more serious difficulties. Because the moist materials tend to cling together they must be driven through the screens under pressure. The moist fibers wrap themselves around the wires that form the mesh, and the screen quickly becomes clogged. Frequent cleaning operations are thus required. Equipment must be designed for ease of dismantling and cleaning, and unless all parts of the equipment are thoroughly cleaned each time there is always the chance of contamination due to residue that may remain over an extended period of time.
The difficulties experienced in the screening of such products are matched by the urgent desirability of screening certain products that seem to offer the most serious challenges. One such product that can benefit significantly from an effective and efficient screening process is ground meat. In this case a screening process is needed for removing cartiledge, gristle and bone fragments from the meat. As known the bone fragments can readily fracture a person's teeth or at the least cause severe pain when bitten into. In spite of all reasonable care taken to eliminate such unwanted portions of the product ahead of the grinding operation a considerable amount of cartiledge, gristle and bone fragments inevitably find their way into the finished product.
The present invention is directed toward the realization of an effective screening process for ground meats and similar materials. The invention employs a different approach to screening, taking advantage of physical differences (other than dimensional) between the meat and the cartiledge, gristle and bone particles specifically the softness and pliability of the fleshy portions as compared with the rigidity and/or semi pliability of the other identified fragments. The separation process employed is a form of extrusion in which a given particle size of meat will pass through the screen while a cartiledge, gristle and bone fragment, of equal or smaller dimensions will be retained and removed.